为什么划线没有显示出来啊(初学者请问)

为什么划线没有显示出来啊(菜鸟请教)
程序代码如下:
unit   xian;

interface

uses
    Windows,   Messages,   SysUtils,   Variants,   Classes,   Graphics,   Controls,   Forms,
    Dialogs,   StdCtrls;

type
    TForm1   =   class(TForm)
        Button1:   TButton;
        procedure   FormMouseDown(Sender:TObject;Button:TMouseButton;
        Shift:TShiftState;X,Y:integer);
        procedure   FormCreate(Sender:   TObject);
        procedure   Button1Click(Sender:   TObject);
    private
        {   Private   declarations   }
    public
        {   Public   declarations   }
    end;

var
    Form1:   TForm1;
    start:boolean;

implementation

{$R   *.dfm}

procedure   TForm1.FormMouseDown(Sender:TObject;Button:TMouseButton;
Shift:TShiftState;X,Y:integer);
begin
if   start   then
form1.Canvas.LineTo(x,y);
end;

procedure   TForm1.FormCreate(Sender:   TObject);
begin
start:=false;
end;

procedure   TForm1.Button1Click(Sender:   TObject);
begin
start:=true;
end;

end.


小弟刚学delphi,很多不懂的，请各位大侠能指教,让菜鸟高飞吧

[解决办法]
unit Unit1;

interface

uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, StdCtrls;

type
TForm1 = class(TForm)
Button1: TButton;
procedure FormCreate(Sender: TObject);
procedure FormMouseDown(Sender: TObject; Button: TMouseButton;
Shift: TShiftState; X, Y: Integer);
procedure Button1Click(Sender: TObject);
private
bStart : Boolean;
iX, iY : Integer;
public
end;

var
Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.FormCreate(Sender: TObject);
begin
iX := 0;
iY := 0;
bStart := False;
end;

procedure TForm1.FormMouseDown(Sender: TObject; Button: TMouseButton;
Shift: TShiftState; X, Y: Integer);
begin
if bStart then
begin
Self.Canvas.MoveTo(iX, iY);
Self.Canvas.LineTo(X, Y);
end;
iX := X;
iY := Y;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
bStart := not bStart;
end;

end.

[解决办法]
可以阿，你的代码在我这里执行挺好的，不过画的是弯弯曲曲的鼠标轨迹线
[解决办法]
各个事件都对好应好代码了吗？如 Form1的OnMouseDown事件里是否对应了相应的代码